Stefan Mischook has made his entire video series detailing the creation of a simple shopping cart in PHP available for download:

After teasing you guys for a week or so, I've released the entire video tutorial course on building a PHP based shopping cart. There is a small charge ($19.99) to download the entire course, but it is more than worth it since you get 140 minutes of top notch nerd training...and we include the source code too.

The course walks you through the creation of the cart - what technology you'll need, building the simple catalog page, making the "add to cart" page, as well as the cart itself (including calculating the costs and shipping costs). It also includes an integration method for hooking into PayPal's IPN setup to change the customer for the goods.